Saving Control Limits

[See SHWXS1 in the SAS/QC Sample Library]You can save the control limits for and charts in a SAS data set; this enables you to apply the control limits to future data (see Reading Preestablished Control Limits) or modify the limits with a DATA step program.

The following statements read measurements from the data set Turbine (see Creating Charts for Means and Standard Deviations from Raw Data) and save the control limits displayed in Figure 13.40.6 in a data set named Turblim:

proc shewhart data=Turbine;
   xschart KWatts*Day / outlimits=Turblim
                        nochart;
run;

The OUTLIMITS= option names the data set containing the control limits, and the NOCHART option suppresses the display of the charts. The data set Turblim is listed in Figure 13.40.10.

The data set Turblim contains one observation with the limits for process KWatts. The variables _LCLX_ and _UCLX_ contain the lower and upper control limits for the chart, and the variables _LCLS_ and _UCLS_ contain the lower and upper control limits for the chart. The variable _MEAN_ contains the central line for the chart, and the variable _S_ contains the central line for the chart. The value of _MEAN_ is an estimate of the process mean, and the value of _STDDEV_ is an estimate of the process standard deviation . The value of _LIMITN_ is the nominal sample size associated with the control limits, and the value of _SIGMAS_ is the multiple of associated with the control limits. The variables _VAR_ and _SUBGRP_ are bookkeeping variables that save the process and subgroup-variable. The variable _TYPE_ is a bookkeeping variable that indicates whether the values of _MEAN_ and _STDDEV_ are estimates or standard values. For more information, see OUTLIMITS= Data Set.

You can create an output data set containing both control limits and summary statistics with the OUTTABLE= option, as illustrated by the following statements:

proc shewhart data=Turbine;
   xschart KWatts*Day / outtable=Turbtab
                        nochart;
run;

The data set Turbtab is listed in Figure 13.40.11.

The data set Turbtab contains one observation for each subgroup sample. The variables _SUBX_, _SUBS_, and _SUBN_ contain the subgroup means, subgroup standard deviations, and subgroup sample sizes. The variables _LCLX_ and _UCLX_ contain the lower and upper control limits for the chart. The variables _LCLS_ and _UCLS_ contain the lower and upper control limits for the chart. The variable _MEAN_ contains the central line for the chart. The variable _S_ contains the central line for the chart. The variables _VAR_ and Batch contain the process name and values of the subgroup-variable, respectively. For more information, see OUTTABLE= Data Set.

A data set created with the OUTTABLE= option can be read later as a TABLE= data set. For example, the following statements read Turbtab and display charts (not shown here) identical to those in Figure 13.40.6:

title 'Mean and Standard Deviation Charts for Power Output';
proc shewhart table=Turbtab;
   xschart KWatts*Day;
run;

Because the SHEWHART procedure simply displays the information in a TABLE= data set, you can use TABLE= data sets to create specialized control charts (see Specialized Control Charts). For more information, see TABLE= Data Set.